Problem: Simplify the following expression and state the conditions under which the simplification is valid. You can assume that $p \neq 0$. $y = \dfrac{4p - 24}{p^3 - 13p^2 + 42p} \div \dfrac{p - 5}{4p^3 - 52p^2 + 168p} $
Answer: Dividing by an expression is the same as multiplying by its inverse. $y = \dfrac{4p - 24}{p^3 - 13p^2 + 42p} \times \dfrac{4p^3 - 52p^2 + 168p}{p - 5} $ First factor out any common factors. $y = \dfrac{4(p - 6)}{p(p^2 - 13p + 42)} \times \dfrac{4p(p^2 - 13p + 42)}{p - 5} $ Then factor the quadratic expressions. $y = \dfrac {4(p - 6)} {p(p - 7)(p - 6)} \times \dfrac {4p(p - 7)(p - 6)} {p - 5} $ Then multiply the two numerators and multiply the two denominators. $y = \dfrac {4(p - 6) \times 4p(p - 7)(p - 6) } { p(p - 7)(p - 6) \times (p - 5)} $ $y = \dfrac {16p(p - 7)(p - 6)(p - 6)} {p(p - 7)(p - 6)(p - 5)} $ Notice that $(p - 7)$ and $(p - 6)$ appear in both the numerator and denominator so we can cancel them. $y = \dfrac {16p\cancel{(p - 7)}(p - 6)(p - 6)} {p\cancel{(p - 7)}(p - 6)(p - 5)} $ We are dividing by $p - 7$ , so $p - 7 \neq 0$ Therefore, $p \neq 7$ $y = \dfrac {16p\cancel{(p - 7)}(p - 6)\cancel{(p - 6)}} {p\cancel{(p - 7)}\cancel{(p - 6)}(p - 5)} $ We are dividing by $p - 6$ , so $p - 6 \neq 0$ Therefore, $p \neq 6$ $y = \dfrac {16p(p - 6)} {p(p - 5)} $ $ y = \dfrac{16(p - 6)}{p - 5}; p \neq 7; p \neq 6 $